By Marc Rayman

As NASA’s Dawn spacecraft investigates its first target, the giant asteroid Vesta, Marc Rayman, Dawn’s chief engineer, shares a monthly update on the mission’s progress.

This artist’s concept shows NASA’s Dawn spacecraft orbiting the giant asteroid Vesta. The depiction of Vesta is based on images obtained by Dawn’s framing cameras. Image credit: NASA/JPL-Caltech |
› Full image and caption

Dear Ups and Dawns,

Dawn is continuing its exploits at Vesta, performing detailed studies of the colossal asteroid from its low altitude mapping orbit (LAMO). The robotic ambassador is operating extremely well on behalf of the creatures it represents on a distant planet. On this second intercalary day of its ambitious adventure, the spacecraft is doing exactly what it was designed to do: exploring a previously uncharted alien world.

Although we usually describe LAMO as being at an average altitude of 210 kilometers (130 miles), that does not mean it is at a constant altitude. As we saw on the fourth anniversary of Dawn’s departure from Earth, there are two reasons the spacecraft’s height changes. One is that the elevation of the surface itself changes, so if the probe flew in a perfect circle around Vesta, its altitude would vary according to the topography. Like the planet from which Dawn embarked upon its deep space journey in 2007 (and even some of the residents there), Vesta is broadest near its equator, and that is where the ground generally reaches its greatest distance from the center. In addition, the ancient surface, battered over billions of years in the rough and tumble of the asteroid belt, displays remarkable variations in shape. The giant Rheasilvia basin is a scar from an extraordinary impact that excavated a region encompassing the south pole more than 500 kilometers (over 300 miles) in diameter. This immense gouge has left that part of Vesta at a much lower elevation than elsewhere. In the center of the enormous depression is the second tallest mountain known in the solar system, soaring to well over twice the height of Mt. Everest. The vertical range from the highest locations near the equator to the bottoms of the deepest craters within Rheasilvia is more than 60 kilometers (37 miles). So as Dawn loops around in just over four hours, the surface underneath it rises and falls dramatically.

The second reason is that the orbit itself is not exactly a circle. Let’s ignore for a moment the effect of the topography and focus solely on the shape of the craft’s path around Vesta. As Vesta rotates and Dawn revolves, the gravitational forces acting on the orbiter are always changing because of the irregular distribution of material inside the geologically complex protoplanet. This effect occurred at the higher altitudes as well, but it was much less pronounced there. Now that the adventurer is deep in the gravity field, the peaks and valleys of its own motion are magnified.

Navigators were very careful in choosing the parameters for LAMO, recognizing that the orbital waters were turbulent. Nevertheless, their mapping of the gravitational currents proved quite accurate, and the spacecraft has followed the planned course quite well. The lengthy and relatively technical discussions in the two previous logs described why the ship drifts off a little, but operators occasionally nudge it back with the ion propulsion system.

Orbits usually are best described by ellipses, like flattened circles. Now Vesta’s bumpy gravity field does not allow perfectly smooth, regular orbits at low altitude. Moreover, the variations in the strength of the gravitational attraction transform the orbits. Sometimes, the difference between the high point of a loop and the low point is less than 16 kilometers (10 miles). As the changing forces reshape the orbit, the ellipse gets more exaggerated, with the low points going lower and the high points going higher. The differences within one revolution grow to be more than 75 kilometers (47 miles). Thanks to the ingenious design of the orbital trajectory however, those same forces then will gradually attenuate the profile, causing it to become more round again. This pattern repeats every 11.5 days in LAMO. It is almost as if the orbit breathes slowly, its envelope expanding and contracting.

› Continue reading Marc Rayman’s Dawn Journal

By Marc Rayman

NASA’s Dawn spacecraft is less than five months away from getting into orbit around its first target, the giant asteroid Vesta. Each month, Marc Rayman, Dawn’s chief engineer, shares an update on the mission’s progress.

Artist’s concept of NASA’s Dawn spacecraft at the large asteroid Vesta. The mission is less than five months away from getting into orbit around the large asteroid, its first target.

Dear Pleasant Dawnversions,

Deep in the asteroid belt, Dawn continues thrusting with its ion propulsion system. The spacecraft is making excellent progress in reshaping its orbit around the sun to match that of its destination, the unexplored world Vesta, with arrival now less than five months away.

We have considered before the extraordinary differences between Dawn’s method of entering orbit and that of planetary missions employing conventional propulsion. This explorer will creep up on Vesta, gradually spiraling closer and closer. Because the probe and its target already are following such similar routes around the sun, Dawn is now approaching Vesta relatively slowly compared to most solar system velocities. The benefit of the more than two years of gentle ion thrusting the spacecraft has completed so far is that now it is closing in at only 0.7 kilometers per second (1600 mph). Each day of powered flight causes that speed to decrease by about 7 meters per second (16 mph) as their orbital paths become still more similar. Of course, both are hurtling around the sun much faster, traveling at more than 21 kilometers per second (47,000 mph), but for Dawn to achieve orbit around Vesta, what matters is their relative velocity.

It may be tempting to think of that difference from other missions as somehow being a result of the destination being different, but that is not the case. The spiral course Dawn will take is a direct consequence of its method of propelling itself. If this spacecraft were entering orbit around any other planetary body, it would follow the same type of flight plan. This unfamiliar kind of trajectory ensues from the long periods of thrusting (enabled by the uniquely high fuel efficiency of the ion propulsion system) with an extremely gentle force.

Designing the spiral trajectories is a complex and sophisticated process. It is not sufficient simply to turn the thrust on and expect to arrive at the desired destination, any more than it is sufficient to press the accelerator pedal on your car and expect to reach your goal. You have to steer carefully (and if you don’t, please don’t drive near me), and so does Dawn. As the ship revolves around Vesta in the giant asteroid’s gravitational grip, it has to change the pointing of the xenon beam constantly to stay on precisely the desired winding route to the intended science orbits.

Dawn will scrutinize Vesta from three different orbits, known somewhat inconveniently as survey orbit, high altitude mapping orbit (HAMO), and low altitude mapping orbit (LAMO). Upon concluding its measurements in each phase, it will resume operating its ion propulsion system, using the mission control team’s instructions for pointing its thruster to fly along the planned spiral to the next orbit.

› Continue reading Dawn Spacecraft Creeping Up on Vesta